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Calibration of satellite altimetry

The development of satellite technology has revealed an increased need for tide gauge data. Radar altimeters on board satellites currently reach near-centimetre accuracy for measuring sea level. However, the satellite measurement must be linked to a measurement from a tide gauge located vertical to the orbit due to the sampling method and the poor knowledge of the near-shore geoid. In this case, and for the study of long-term changes in sea level, the geodetic positioning of tide gauges is an absolute necessity.

How satellite altimetry works

Satellite altimetry is based on the principle of radar: a transmitter and a receiver on a satellite measure round trip travel time, along the vertical path, of electromagnetic pulses reflected from the surface of the sea. The time can be translated into distance based on propagation speed, providing the height of the satellite above the surface with accuracy of a few centimetres. If we know the position of the satellite, we can deduce the position of the surface.

However, there are a number of inherent problems in using the technology and the data provided.

Precise positioning of the satellite

This is done by laser ranging from stations located along the satellite track. These stations must be located in a coherent reference system. Modern space geodesy technology can position specific points on the surface of the Earth with centimetre accuracy in the geocentric reference system International Terrestrial Reference System (IRTS) adopted by the International Union for Geodesy and Geophysics (IUGG). Because the stations are positioned in this system, it follows that the sea level measured using this technology is linked to this reference frame. This raises the problem of positioning with respect to the usual references (land levelling, mean level, chart datum). This is why stations are usually located near tide gauges.

Altimeter calibration

Like any indirect measurement instrument, altimeters must be calibrated. This consists of comparing the altimeter measurement to a direct measurement provided by a tide gauge. But the first exploitable altimeter measurement is several kilometres away from the coast, which raises the problem of the slope of the water between the tide gauge located on land and the first exploitable altimeter measurement.

Using the results

Altimeters are placed on board sun-synchronous satellites, which means they pass along the same track at regular intervals (a few days apart). The tracks are spaced a few hundred kilometres apart and oriented obliquely relative to the axis of the poles. Ascending tracks (South-North) cross descending tracks (North-South), forming a regular grid. Measurements at a given point are therefore available only at time intervals greater than the fundamental periods of the tide, which makes it impossible to analyse them using conventional methods. But, with appropriate methods, the lack of time information at a point is offset in part by the abundance of spatial information.

Altimetry measurements provide very interesting results in the field of oceanography and geodesy.

For tides, current applications include:

improving the accuracy of cotidal line charts worldwide,

the inclusion of these data in numerical models using "assimilation" techniques,

evaluating the rise in mean sea level.

This technique is of obvious interest, but has its limits: the need for a reference tide gauge station, no usable data near the coast, the problem of geodesic linking far from reference stations and the difficulty of processing the data.